Bonding fiber for airlaid multi-layer products and process for production of said airlaid multi-layer products

ABSTRACT

A bonding fiber and a process using this fiber have been developed to produce airlaid multi-layer structures with sufficient bonding within each layer and between layers including bonding to unwound carrier sheets using a bonding fiber which will remain in the intended layer of the airlaid multi-layer structure. Further the bonding fiber will not be lost through low basis weight unwound carrier and top sheets. A bonding fiber for airlaid, multilayer products comprising a thermoplastic material selected from the group comprising nanofibers, micro-fibers, and 0.5 to 30 dTex staple fibers and having a length between 0.3 mm to 70 mm.

FIELD OF THE INVENTION

The present invention relates to airlaid multi-layer products in general and the method for producing said airlaid multi-layer products.

These airlaid multi-layer products are of use in end uses such as diapers, feminine hygiene products, meat pads, specialty wipes, and table cloth with liquid barrier, multi-layer heat and sound insulation.

BACKGROUND OF THE INVENTION

Multi-layer products using airlaid nonwovens are susceptible to delamination during production, converting or during end use. Each layer can consist of a carrier sheet or top sheet in combination with inline formed airlaid layers. In order to avoid delamination, bonding agents are added in the different layers of the airlaid structure. Examples of bonding agents include latex and polyethylene (PE) powder.

If a PE powder is used in the inline formed airlaid layers it can, during the forming process, migrate through the layers of the web and thereby change the composition of each layer. This reduces the amount available for the bonding function between the layers. Furthermore, it can thereafter leak out of the airlaid multi-layer web, through the air permeable forming wire and into the air filter systems, block the air permeable forming wire and stick to different machine parts. If an unwound carrier sheet is part of the airlaid multi-layer product it will act as a barrier given that that carrier material is of a structure that can detain the PE powder. With the increasing tendency to down gauge, or even eliminate, carrier sheets, the carrier sheet can be too low basis weight to effectively detain the PE powder.

The need exists for bonding agents that will stay in the intended layer of the airlaid multi-layer structure and a bonding agent that will not be lost through low basis weight unwound carrier sheets.

It is therefore an object of the present invention to provide a bonding agent which will remain in the layer it has been airlaid formed into and not be lost through low basis weight carrier or top sheets. It is a further object of the present invention to provide a bonding agent that results in an airlaid multi-layer product having sufficient bonding within and between layers to prevent delamination.

SUMMARY OF THE INVENTION

A bonding fiber and a process using this fiber have been developed to produce airlaid multi-layer structures with sufficient bonding within each layer and between layers including bonding to unwound carrier sheets using a bonding fiber which will remain in the intended layer of the airlaid multi-layer structure. Further the bonding fiber will not be lost through low basis weight unwound carrier and top sheets.

The bonding fiber of the present invention is a thermoplastic material in the form of a fiber such as nanofibers, micro-fibers or 0.5 to 30 dTex staple fiber. The fiber has a length from 0.3 mm to 70 mm.

DETAILED DESCRIPTION

In order to control bonding within each of the layers of an airlaid multi-layer structure and between layers of the structure including bonding to carrier and top sheets a fiber of the present invention as described above is used in the airlaid forming systems. Such fibers can be used in homogeneous blends with any other raw materials used, between the carrier or top sheet and the contacting airlaid formed layer or as a 100% fiber layer between each layer of the airlaid multi-layer product.

Common size of particles in PE powders is 100-500 microns, for example 300 microns. The number of fibers, taking into account different dTex and fiber lengths, that are of equivalent mass as a common size PE particle is given below. The higher number of fibers is expected to result in improved bonding efficiency i.e. higher tensile strength.

n Fiber Length (mm) dtex 3 3.5 4 2.2 20 17 15 3.3 14 12 10 11 4 3 3 30 1 1 1

Bold figures give the number of fibers at the dTex and fiber length of the table at the weight of a PE particle.

Because of the smaller lateral dimension of the fibers they will melt relatively easily in the airlaid process permitting a broader range of raw materials as well as increased production speeds. To exemplify: While PE powders have to be of high melt flow index (MFI) to perform in the airlaid process the fibers of the present invention do not need to have same high MFI to permit melting and thus bonding within the time-temperature constraints of an airlaid process.

We propose that the fibers of the present invention can be mono-component fibers as well as bi-component fibers made from raw materials of the polyolefin family. In particular we propose the raw material to be chosen from the range of ethylene and propylene based polymers.

While bi-component fibers are known in the art for airlaid products, they are designed only to bond to pulp within the airlaid layers, or sometimes to bond one airlaid layer to another. The fibers are not designed to bond an airlaid layer to a carrier or top sheet. The mono-component fibers can be chosen to maximize bonding strength under commercial conditions along with other criteria such as economics, dryer conditions, etc.

The following examples will serve to illustrate the invention.

EXAMPLES

A PE based bi-component bonding fiber, suitable for airlaid processes, was produced as a 2.1 dTex 3 mm short cut fiber with tenacity of 2.1 cN/dTex and elongation of 188%. The fiber was tested at an airlaid testing facility producing a non-woven (NW) airlaid formed onto an unwound carrier sheet targeting a basis weight of 100 gsm and a caliper of 1.1 mm. Further the NW was compacted to a minimum and resulting caliper subjected for investigation. The fiber was formed as a homogeneous blend with pulp through an 8 mesh screen and bonded at 148° C. The control fibers were made with polyethylene only whereas the fibers in Example 1 were made as bicomponent fibers with PE polymer in the core and the sheath, with PP added to the core.

Tensile testing: N=10; draw speed=100 mm/min; sample size=50×100 mm; carrier sheet has been removed prior to testing.

Results:

Basis weight Caliper Tensile Elongation Sample ID [gsm] [mm] [cN/dTex] [%] Control 98 1.06 2.1 5.9 Example 1 98 0.75 4.7 4.2

Forming resulted in a uniform mat as a result of good blending with pulp and no issues with respect to run-ability in the airlaid systems.

The pulp from the nonwoven webs was extracted with acid leaving behind only the bonding fibers. The nonwovens were imaged to establish the difference between fibers of the invention and control fibers.

The photographs in FIG. 1 compare the extracted fabrics made with control PE fibers (left) and from example 1 (right). The improved uniformity is evident.

The electron microscope images of FIG. 2 show the comparison of the control fibers and the fibers of the invention. The difference between the control and fibers of this invention is clearly evident. The fabrics made with the control fibers (left) tend to show more globule formation compared to the fabrics from fibers from the invention (right). 

What is claimed is:
 1. A bonding fiber for airlaid, multilayer products comprising a thermoplastic material selected from the group comprising nanofibers, micro-fibers, and 0.5 to 30 dTex staple fibers and having a length between 0.3 mm to 70 mm. 